Method for decontaminating surfaces

ABSTRACT

A method chemically decontaminates a surface of a metal component of a primary circuit of a pressurized-water reactor, the surface contains an oxide layer. In a first stage, an oxidation step, a reduction step, and a first decontamination step are performed. The component is treated in the oxidation step with an aqueous solution containing an oxidizing agent, which converts trivalent chrome present in the oxide layer into hexavalent chrome. The component is treated in the reduction step with an aqueous solution containing a reducing agent for reducing excess oxidation agent from the oxidation step. The component is treated in the first decontamination step with an aqueous solution containing a decontamination acid that forms no antisoluble deposits with metal ions in the solution. The solution is fed through an ion exchanger for removing metal ions. In a second stage, the component is treated with an aqueous solution containing an oxalic acid.

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation application, under 35 U.S.C. §120, of copending international application No. PCT/EP2010/068602, filed Dec. 1, 2010, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of German patent application No. DE 10 2009 047 524.9, filed Dec. 4, 2009; the prior applications are herewith incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The invention relates to a method for decontaminating surfaces of components of the coolant circuit of a pressurized water reactor. The key element of the coolant circuit is a reactor pressure vessel in which fuel elements containing the reactor fuel are situated. Multiple cooling loops, each having a coolant pump and a steam generator, are usually connected to the reactor pressure vessel.

Under the conditions of on-load operation of a pressurized water reactor at temperatures in the range of 288° C., even stainless austenitic FeCrNi steels, of which the piping system of the cooling loops, for example, is composed, Ni alloys, of which the exchanger tubes of steam generators, for example, are composed, and other components containing cobalt, for example, for such elements as coolant pumps, have a certain solubility in water. Metal ions which leach from the referenced alloys are carried by the coolant flow to the reactor pressure vessel, where they are partially converted into radioactive nuclides as the result of neutron radiation which is present at that location. The nuclides are then distributed through the entire coolant system by the coolant flow, and are deposited in oxide layers which form on the surfaces of components of the coolant system during operation. Over extended operating periods the quantity of deposited activated nuclides accumulates, resulting in an increase in the radioactivity, i.e., the dose rate, of the components of the coolant system. Depending on the type of alloy used for a component, the oxide layers contain as the primary component iron oxide with bi- and trivalent iron and oxides of other metals, primarily chromium and nickel, which are present as alloy components in the above-mentioned steels. Nickel is always present in the bivalent form (Ni²+), and chromium, in the trivalent form (Cr³+).

Before inspection, maintenance, repair, and dismantling procedures can be carried out on the coolant system, it is necessary to reduce the radioactive radiation of the components in question in order to decrease the level of personal radiation exposure. This is achieved by removing as much as possible of the oxide layer which is present on the surfaces of the components, using a decontamination method. In such decontamination, either the entire coolant system or a portion which is separated therefrom by valves, for example, is filled with an aqueous cleaning solution, or individual components of the system are treated in a separate container which contains the cleaning solution. For chromium-containing components, for example in the case of a pressurized water reactor, the oxide layer is first oxidatively treated (oxidation step), and the oxide layer is subsequently dissolved under acidic conditions in a so-called decontamination step using an acid, referred to below as decontamination acid or decon acid. The metal ions which pass from the oxide layer into the solution may then be removed from the solution by leading them through an ion exchanger. Excess oxidizing agent from the oxidation step is neutralized, i.e., reduced, in a reduction step by adding a reducing agent. Thus, the dissolution of the oxide layer or the leaching of metal ions in the decontamination step occurs in the absence of an oxidizing agent. The reduction of the excess oxidizing agent may be an independent treatment step, whereby a reducing agent which is used only for the purpose of reduction, for example ascorbic acid or citric acid, or hydrogen peroxide for the reduction of permanganate ions and manganese dioxide, is added to the cleaning solution. However, excess oxidizing agent may also be reduced within the scope of the decontamination step, using, in addition to the reducing agent, a decontamination acid which causes the oxide layer to dissolve, or an acid which is able to reduce excess oxidizing agent, for example the frequently used permanganate ion and the resulting manganese dioxide. In the mentioned case, a quantity of decontamination acid which is sufficient on the one hand to neutralize excess oxidizing agent and on the other hand to dissolve the oxide is added to the solution. As a rule, the treatment sequence “oxidation step-reduction step-decontamination step” or “oxidation step-decontamination step with simultaneous reduction” is applied multiple times to achieve the desired result. The same decon acid or mixture of decon acids is always used in the decontamination step.

The oxidative treatment of the oxide layer is necessary due to the fact that chromium(III) oxides and mixed oxides, primarily of the spinel type, containing trivalent chromium are only sparingly soluble in the acids which are suitable for decontamination. For this reason, to increase the solubility the oxide layer is first treated with an aqueous solution of an oxidizing agent such as Ce⁴⁺, HMnO₄, H₂S₂O₈, KMnO₄, KMnO₄ with acid or base, or O₃. As a result of this treatment, Cr(III) is oxidized to Cr(VI), which goes into solution as CrO₄ ²⁻.

Due to the presence of a reducing agent in the decontamination step, the Cr(VI) which is produced in the oxidation step and which is present as chromate in the cleaning solution is reduced back to Cr(III). At the end of a decontamination step, the cleaning solution contains Cr(III), Fe(II), Fe(III), and Ni(II), in addition to radioactive isotopes such as Co-60. These metal ions may be removed from the cleaning solution using an ion exchanger. A frequently used decon acid in the decontamination step is oxalic acid due to its ability to dissolve oxide layers to be removed from component surfaces.

However, it is disadvantageous that oxalic acid together with bivalent metal ions such as Ni²⁺, Fe²⁺, Co²⁺, and Cu²⁺ forms sparingly soluble oxalate precipitates which become distributed through the entire coolant system and deposit on the inner surfaces of pipes and of components, for example steam generators. As a result, the precipitates complicate carrying out the overall method. For this reason, organic constituents of a solution are often converted to carbon dioxide and water by treatment with an oxidizing agent and UV irradiation, and are thus removed from the solution. However, the precipitates cause turbidity of the solution, which significantly reduces the effectiveness of the UV irradiation. In addition, this results in co-precipitation of radionuclides, and thus, recontamination of the component surfaces. The risk of recontamination is particularly high for components having a large surface-to-volume ratio. This is the case for steam generators in particular, which have a very large number of small-diameter exchanger tubes. Another disadvantage of the use of oxalic acid is that oxalate precipitates may plug filter units, such as the filters and sieve plates provided upstream from an ion exchanger, or the protective filters of circulation pumps. Lastly, a further disadvantage results when an above-described treatment cycle composed of an oxidation step and a decontamination step is repeated, i.e., when a decontamination step is followed by another oxidation step. If oxalate precipitates were produced in the preceding decontamination step, the corresponding metal ions, such as Ni in the case of a nickel oxalate precipitate, cannot be removed from the solution by using ion exchangers. As a result, in the subsequent oxidation step the oxalate residue of the precipitates is oxidized to form carbon dioxide and water, and therefore oxidizing agent is needlessly consumed. On the other hand, if the oxalate is present in solution, i.e., not bound in the form of a precipitate, the oxalate may be easily and cost-effectively decomposed by UV light, for example, i.e., converted to carbon dioxide and water, for example before the cleaning solution is led into an ion exchanger. Lastly, a further disadvantage is that turbidity caused by an oxalate precipitate interferes with monitoring of the process, using photometry, for example.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a method for decontaminating surfaces which overcomes the above-mentioned disadvantages of the prior art methods of this general type.

This object is achieved by a decontamination method which is divided into two process stages.

In the first process stage at least one treatment cycle is performed, containing an oxidation step, a reduction step, and a subsequent first decontamination step. Depending on the extent and type of oxide formation on the component surfaces, such a treatment cycle can be performed only once, or also multiple times. In the oxidation step the component is treated with an aqueous cleaning solution which contains an oxidizing agent whose oxidizing power is sufficient to convert the trivalent chromium contained in the oxide layer to hexavalent chromium. As previously described, as a result of this step the solubility of an oxide layer present on the component is increased. In the reduction step the component is treated with a solution containing a reducing agent in order to reduce excess oxidizing agent from the oxidation step. In the first decontamination step the component is treated with an aqueous solution which exclusively or predominantly (i.e., in a proportion greater than 50 mol-%) contains at least one decontamination acid that forms no sparingly soluble precipitates with metal ions present in the solution, in particular bivalent metal ions such as Ni(II), Fe(II), Co(II), and Mn(II), as is the case for oxalic acid, for example. It is practical to use a decon acid which also forms no sparingly soluble precipitates with tribasic and higher basic acids, which, however, is the case for the acids typically used for decontamination of the present type, for example formic acid and glyoxylic acid. The formation of sparingly soluble nickel oxalate precipitates in particular is prevented in this manner. During or at the end of the decontamination step, the solution for removal of metal ions contained therein which originate from the oxide layer and/or the base metal of the component is led through an ion exchanger.

The reduction step and the decontamination step may also be carried out together, i.e., simultaneously, as described above.

Thus, in the proposed manner, in the first process stage a significant portion of the metal ions, primarily Ni(II), Fe(II), and Co(II), which are critical with regard to the formation of sparingly soluble precipitates may be removed from the cleaning solution, and thus, from the component surface to be decontaminated, without the risk of forming sparingly soluble precipitates. In a second process stage there is the option of carrying out a second decontamination step in which highly effective oxalic acid may be easily used, primarily to leach out Fe(III) and Fe(II) present in the oxide layer, since the critical bivalent ions, primarily Ni(II), are no longer present or are present in a concentration in the cleaning solution which no longer results in precipitates. Thus, in the method according to the invention two different decontamination variants are used, whereby in the first variant or the first decontamination step, ions which form sparingly soluble oxalate precipitates are removed, and remaining ions such as Fe(III) and Fe(II) may subsequently be brought into solution using oxalic acid, which is highly effective with regard to oxide dissolution. It is irrelevant per se whether the dissolution of Fe(II) or Fe(III) from the oxide layer, brought about by the “noncritical” decontamination acid used in the first process stage, is effective, since this may be effectively carried out in the second process stage using oxalic acid.

Preferably only oxalic acid is used in the second decontamination step. However, a mixture of one or more other decon acids in which oxalic acid predominates, i.e., is present in a proportion greater than 50 mol-%, is also conceivable.

In summary, a method according to the invention provides the option of preventing or at least greatly reducing the formation of sparingly soluble precipitates without decreasing the effectiveness of the decontamination.

The method may be carried out in such a way that in the first process stage, at least one treatment cycle is first carried out, and in the subsequent second process stage the component surface is treated without a preceding oxidation in the second decontamination step; i.e., the oxide layer of the component is treated with oxalic acid. However, it is also conceivable that in the second process stage the oxide layer is first treated, for example using the above-mentioned oxidizing agents, and only then is the oxide layer dissolution with oxalic acid carried out. In this case, of course, a reduction step as described above is also necessary.

An organic acid is preferably used in the first decontamination step, since its organic component—provided that the acid consists of carbon, hydrogen, and oxygen—may be converted to carbon dioxide and water and thus removed with practically no residue, since the carbon dioxide escapes from the solution as a gas. The organic constituents are removed in a manner known per se by irradiating the solution, to which an oxidizing agent such as hydrogen peroxide is added, with UV light. Acids are preferably used which consist exclusively of carbon, hydrogen, and oxygen, so that no residues, resulting from elements such as nitrogen, remain behind which are removable only with the aid of ion exchangers, and which therefore result in the generation of secondary waste (additional exchanger material which must be disposed of).

In some countries such as Japan, the loading of ion exchangers with complex-forming acids or complexes of such acids in the course of decontamination measures of the present type is not allowed. Therefore, in these cases it is advantageous to use acids which do not form complexes with metal ions.

An acid containing a maximum of two carbon atoms is preferably used in the first decontamination step. The decomposition of such an acid to form carbon dioxide and water takes place more rapidly than the decomposition of acids containing three or more carbon atoms, so that time, energy, and oxidizing agent, and ultimately also costs, may be saved.

Examples of acids which are suitable for the decontamination step in the first process stage include inorganic acids such as HNO₃, HBF₄, and H₂SO₄, noncomplex-forming monocarboxylic acids such as formic acid, acetic acid, monohydroxyacetic acid, and dihydroxyacetic acid, and complex-forming acids such as EDTA, nitrilotriacetic acid, and tartronic acid. Formic acid and glyoxylic acid have proven to be suitable for waste prevention, the best decontamination factors being achieved when only glyoxylic acid is used in the first process stage. These acids form a soluble salt with the metal ions, in particular with the nickel of the oxide layer. When such a salt-containing solution is led through a cation exchanger, the metal ion is retained and the acid anions remain in solution, and, as described above, may be subsequently decomposed by oxidation in a residue-free manner. This is not the case for glycine, for example, which contains a nitrogen atom, or for the inorganic acids.

Other features which are considered as characteristic for the invention are set forth in the appended claims.

Although the invention is described herein as embodied in a method for decontaminating surfaces, it is nevertheless not intended to be limited to the details described, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.

The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying tables.

DETAILED DESCRIPTION OF THE INVENTION

To verify the effectiveness of the proposed method, tests were conducted using samples from a primary circuit of a pressurized water reactor (see Table 1). The samples were immersed in a 1-liter container containing a cleaning solution at a temperature of approximately 90° C. As described above, in a decontamination method the metal ions leached from an oxide layer are removed from the cleaning solution using an ion exchanger. For simplicity, ion exchange was not performed in the tests; instead, the particular cleaning solution was discarded at the end of a treatment cycle (oxidation step and decontamination step) and replaced with a new cleaning solution. All of the tests described below were conducted in the acidic range of approximately pH 2.

Three different method variants, each composed of three treatment cycles, were carried out using the samples according to Tables 1 through 3. Each treatment cycle included an oxidation step and a decontamination step. For oxidation of the oxide layer, the container containing the samples was filled with an HMnO₄ solution (concentration=240 ppm). The exposure time was 16 hours in each case. In the first two cycles, formic acid and/or glyoxylic acid, not oxalic acid, was used for the decontamination step (see Tables 1-3). After each oxidation step, excess oxidizing agent (HMnO₄) was neutralized by adding an appropriate amount of reducing agent, followed by addition of the particular acid used in the decontamination step. The time of exposure to the acid in the decontamination step was 5 hours in each case.

TABLE 1 Variant 1 using sample TA-03-2 Oxide layer dissolution (decontamination step) Method stage 1 Cycle 1 50 mmol/L formic acid Cycle 2 25 mmol/L glyoxylic acid Method stage 2 Cycle 3 2000 ppm oxalic acid

TABLE 2 Variant 2 using sample TA-03-3 Oxide layer dissolution (decontamination step) Method stage 1 Cycle 1 25 mmol/L glyoxylic acid Cycle 2 25 mmol/L glyoxylic acid Method stage 2 Cycle 3 2000 ppm oxalic acid

TABLE 3 Variant 3 using sample TA-03-1 Oxide layer dissolution (decontamination step) Method stage 1 Cycle 1 50 mmol/L formic acid Cycle 2 50 mmol/L formic acid Method stage 2 Cycle 3 2000 ppm oxalic acid

For the samples, in each case the Co-60 gamma activity (Becqerel or Bq) present initially and after decontamination step was measured, and the overall decontamination factor (DF), i.e., the ratio of the initial activity to the activity of a sample present after a cycle, was determined. The results are summarized in Table 4.

TABLE 4 Co-60 activity in Bq/sample before/after treatment, and decontamination factors TA-03-2 TA-03-3 TA-03-1 Sample Bq/DF Bq/DF Bq/DF Untreated 5.40E+4 4.48E+4 5.08E+4 Cycle 1 1.32E+4/4.1 1.01E+4/4.4 9.15E+3/5.6 Cycle 2 4.67E+3/11.6 1.61E+3/27.8 1.65E+2/72 Cycle 3 1.38E+2/391 5.78E+1/776 3.07E+1/1654

In the evaluation of the results, it is noted that a decontamination factor of approximately 10 is generally sufficient. Such a factor is already achieved after the second cycle. It is further noted that glyoxylic acid is most effective for the decontamination, i.e., dissolution of the oxide layer, in particular when this acid is used in multiple, preferably all, decontamination cycles in the first process stage.

In the above-described tests which simulate the method according to the invention, the organic acids glyoxylic acid and formic acid were used as examples. However, inorganic acids are also suitable for the decontamination steps of the first process stage. To demonstrate their effectiveness, a test was conducted in which a sample from the primary circuit of a pressurized water reactor having a size corresponding to the above-mentioned samples was subjected to a cycle composed of an oxidation step and a decontamination step. For a cleaning solution volume of 600 mL and a temperature of approximately 95° C., oxidation was first carried out on the oxide layer present on the sample, using HMnO₄ (300 pm), for a period of 20 hours. Residual oxidizing agent present after this step was neutralized with a mixture of hydrogen peroxide and nitric acid, the first component being necessary to dissolve the manganese dioxide (MnO₂) formed from HMnO₄ in the oxidation step. This was followed by a 5-hour decontamination step in which the nitric acid (HNO₃) already contained in solution acted as decon acid, i.e., for dissolving the oxide layer present on the sample. The gamma activity of the sample dropped to a value of 2.18E+4 Bq after the decontamination step. Compared to the initial activity of 6.88E+4 Bq of the sample, this corresponds to a decontamination factor of 3.16. 

1. A method for chemically decontaminating surfaces having an oxide layer of a metallic component of a primary circuit of a pressurized water reactor, the method being divided into two process stages, which comprises the steps of: during a first process stage, performing at least one treatment cycle, including an oxidation step, a reduction step, and a subsequent first decontamination step, the first process stage including the steps of: during the oxidation step, treating the metallic component with a first aqueous solution containing an oxidizing agent for converting trivalent chromium present in the oxide layer to hexavalent chromium; during the reduction step, treating the metallic component with a second aqueous solution containing a reducing agent for reducing excess oxidation agent producing during the oxidation step; during the first decontamination step, treating the metallic component with a third aqueous solution which exclusively or predominantly contains at least one decontamination acid that forms no sparingly soluble deposits with metal ions present in the solution, including bivalent metal ions; leading the third aqueous solution through an ion exchanger for removing the metal ions, present in the third aqueous solution, which originate from the oxide layer or a base metal of the metallic component; and during a second process stage, performing at least one further treatment cycle, including a second decontamination step in which the metallic component is treated with a fourth aqueous solution which exclusively or predominantly contains oxalic acid as a decontamination acid.
 2. The method according to claim 1, which further comprises during the further treatment cycle of the second process stage, performing an oxidation step prior to the second decontamination step.
 3. The method according to claim 1, which further comprises providing an organic acid in the first decontamination step.
 4. The method according to claim 3, which further comprises providing the decontamination acid to consisting exclusively of carbon, oxygen, and hydrogen.
 5. The method according to claim 1, which further comprises providing in the first decontamination step an organic acid which does not form a complex with the metal ions.
 6. The method according to claim 1, which further comprises providing in the first decontamination step at least one decontamination acid containing a maximum of two carbon atoms in a molecule.
 7. The method according to claim 6, which further comprises using at least one of a formic acid or a glyoxylic acid.
 8. The method according to claim 7, which further comprises using the glyoxylic acid in the first decontamination step.
 9. The method according to claim 1, which further comprises neutralizing a residue of the oxidizing agent which is present in the first aqueous solution at the end of the oxidation step with a reducing agent which is added to the first aqueous solution resulting in the second aqueous solution, and the second aqueous solution is used in the subsequent first decontamination step.
 10. The method according to claim 9, which further comprises employing the decontamination acid used in the first decontamination step as the reducing agent. 